[Bug tree-optimization/18694] [4.0 regression] loop miscompilation at -O1 (-ftree-ch)

["law at redhat dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From law at redhat dot com  2004-12-10 20:12 -------
Subject: Re:  [4.0 regression] loop
	miscompilation at -O1 (-ftree-ch)

On Fri, 2004-12-10 at 19:57 +0000, kazu at cs dot umass dot edu wrote:
> ------- Additional Comments From kazu at cs dot umass dot edu  2004-12-10 19:57 -------
> Subject: Re:  [4.0 regression] loop
>  miscompilation at -O1 (-ftree-ch)
> 
> Hi Jeff,
> 
> > > I think so. :-)
> > I don't.  :(  I think it'll record tmp_1 = next_2, which is actually
> > wrong, even though it doesn't actually cause problems with this
> > testcase.
> 
> IMHO, you should really think of it as tmp_1 -> next_2 or "tmp_1 is a
> copy-of next_2".  It is a one-way relation, not an equivalence because
> it is not symmetric.
Err, no.  You're totally warping how the the equivalency code is meant
to work.  It's a symmetric relationship and it's your patch that is
making it asymmetric.


> Let me also think hard to see if I can prove my patch is correct or
> come up with a counterexample.  I certainly don't intend to push my
> patch blindly.
Don't bother.  The patch is wrong.    There's a couple of very simple
ways to fix this problem that I'm evaluating.

Jeff



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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18694